There has been known a technique in which a discharge electrode is arranged in an exhaust passage of an internal combustion engine, and a corona discharge is caused to occur from the discharge electrode, whereby particulate matter (hereinafter also referred to as PM) is charged and condensed or aggregated (see, for example, a first patent document). By the condensation or aggregation of the particulate matter, the number of particles in the particulate matter can be decreased. In addition, the sizes of particles in the particulate matter become large, so when a filter is arranged at a downstream side, it becomes easy to trap the particulate matter with the filter.
However, electricity flows to the electrode through unburnt fuel such as HC, CO, etc., contained in an exhaust gas, too. In the past, no consideration has been given to an electric current passing to the electrode through this unburnt fuel. Then, when a lot of unburnt fuel is contained in the exhaust gas, an electric current passing through the electrode may become large, thus giving rise to a fear that a power supply, the electrode, or other circuits may be deteriorated or failed. In addition, if the apparatus is constructed so as to withstand a large electric current, it will result in an increase in the cost of production. Moreover, when the electric current increases, electric power consumption will become large, so there will also be a fear that fuel economy may get worse.